How to Use This Calculator
Select your post size, then adjust the hole diameter and depth using the sliders. Enter the number of holes you need to fill and the calculator will instantly show you how many bags of concrete to buy. Optionally enter your ZIP code to check whether your hole depth meets your local frost line requirement.
Understanding Footing Requirements
Concrete footings are the foundation that supports deck posts, fence posts, pergola columns, and other structures. A properly sized footing distributes the load into the ground and prevents settling or shifting over time. The two most critical dimensions are the diameter (which determines lateral stability) and the depth (which must reach below the frost line to prevent heaving).
Frost Line Depths
The frost line is the maximum depth at which ground water freezes in winter. When water freezes it expands, pushing footings upward in a process called frost heaving. Building codes require footings to extend below this depth. Frost lines range from 0 inches in southern Florida to 48+ inches in Minnesota and the Dakotas. Enter your ZIP code above to find the estimated frost line for your area.
Concrete Bag Sizes Explained
60lb Bags
Each 60lb bag of concrete mix (such as Quikrete or Sakrete) yields approximately 0.45 cubic feet when mixed. These bags are easier to handle and a good choice when you have a smaller project or limited lifting ability. They cost roughly $4–$5 per bag at most home improvement stores.
80lb Bags
Each 80lb bag yields approximately 0.60 cubic feet. These are the most popular choice for footing projects because they offer a good balance of value and manageability. They cost roughly $5–$6 per bag. For most deck and fence projects, 80lb bags are the recommended option.
Ready-Mix Concrete
Ready-mix concrete is delivered by truck in cubic yards. It makes sense for larger projects (typically 10+ holes) because it saves significant mixing time and labor. Most suppliers have a minimum order of 1 cubic yard, and prices typically range from $125 to $175 per cubic yard depending on your location.
Common Footing Sizes
| Post Size | Recommended Hole Diameter | Typical Depth | Common Use |
|---|---|---|---|
| 4x4 inch | 10–12 inches | 24–36 inches | Fence posts, mailbox posts |
| 6x6 inch | 12–18 inches | 36–48 inches | Deck posts, pergola posts |
| 8x8 inch | 18–24 inches | 42–60 inches | Heavy structures, multi-story decks |
How Much Does Concrete Cost?
As of 2025, typical concrete costs are:
- 60lb bag: $4–$5 each
- 80lb bag: $5–$6 each
- Ready-mix delivery: $125–$175 per cubic yard
Prices vary by region, supplier, and season. The estimates in this calculator use mid-range pricing ($4.50 for 60lb, $5.50 for 80lb, $125 per yard for ready-mix) and include a 10% waste factor for bagged concrete.
Bags vs. Ready-Mix: Which Should You Use?
Use bags when you have fewer than 10 holes, when you want to pour at your own pace, or when truck access to your site is limited. Bags let you mix only what you need and work one hole at a time.
Use ready-mix when you have 10 or more holes, need a large total volume (1+ cubic yards), or want to finish the job faster. Ready-mix saves hours of manual mixing and ensures consistent quality across all footings.
Frequently Asked Questions
How deep should deck footings be?
Deck footings should extend below your local frost line, which ranges from 12 inches in southern states to 48+ inches in northern states. Most building codes require a minimum of 12 inches, but always check your local requirements. Enter your ZIP code above to look up your frost line depth.
Do I need to go below the frost line?
Yes. Footings that don't extend below the frost line can heave during freeze-thaw cycles, causing posts to shift and structures to become unlevel. This can lead to costly repairs. Going below the frost line is required by virtually all building codes.
How many 80lb bags for a 12-inch diameter, 36-inch deep hole?
A 12-inch diameter hole that is 36 inches deep requires about 2.36 cubic feet of concrete. Since each 80lb bag yields 0.60 cubic feet, you need 4 bags (2.36 ÷ 0.60 = 3.93, rounded up). With a 10% waste factor, buy 5 bags to be safe.
Should I use Quikrete or Sakrete?
Both are quality concrete mixes suitable for footings. They have similar yields and strength ratings (around 4,000 PSI). Choose whichever is available at your local home improvement store. This calculator works accurately for both brands as well as other standard concrete mixes.
Can I use leftover concrete?
Unused dry concrete mix can be stored in a sealed bag in a dry location for several months. Once mixed with water, concrete must be used within about 90 minutes before it begins to set. Never add water to concrete that has started hardening — it weakens the final product.
How long does concrete take to cure?
Concrete reaches about 70% of its rated strength in 7 days and full design strength at 28 days. You can typically set posts and attach light loads after 24–48 hours, but wait at least 7 days before applying heavy structural loads like beams or joists.